The long term goal of this research program is to understand the cellular and synaptic mechanisms involved in generating and controlling the mammalian respiratory rhythm. Clearly, breathing is essential to the maintenance of life itself, therefore, understanding the origin and regulation of the respiratory rhythm is of fundamental physiological importance. The respiratory rhythm is generated and controlled by a neuronal circuit within the brainstem but the mechanisms responsible for this rhythm have remained a mystery. Respiratory neurons are concentrated in two groups within the medulla. These groups are called the dorsal respiratory group (DRG) and the ventral respiratory group (VRG). This proposal is a competitive renewal for a project using an in vitro brainstem slice preparation to characterize the cellular and morphological properties of medullary respiratory neurons within these groups. During the previous grant period we refined the brainstem slice preparation and have exploited it to study the properties of neurons in the DRG. The cellular properties of neurons in the VRG, which is the largest group of respiratory neurons in the medulla, are virtually unknown. The VRG extends from the rostral reaches of the spinal cord to the ponto-medullary border and encompasses the nucleus ambiguus and retroambiguus as well as Botzinger's complex. Our specific aims are: 1. To characterize the eloctrophysiological properties of neurons within the VRG using intracellular recording to quantify intrinsic cellular properties and to determine if classes of neurons can be defined by these properties. 2. To characterize the morphological properties of neurons in the VRG by intracellular staining with Lucifer yellow or horseradish peroxidase (HRP). 3. Are the properties of VRG neurons modified by neurotransmitters known to influence respiration? Candidate neurotransmitters include thyrotropin- releasing hormone, serotonin, and substance P. 4. To use dual intracellular recording to quantify synaptic interactions between DRG and VRG neurons.